Piezoelectric behavior of (1−x)K0.50Na0.50NbO3−xBa0.80Ca0.20ZrO3 lead-free ceramics

(1−x)K_0.50Na_0.50NbO₃–xBa_0.80Ca_0.20ZrO₃ [(1−x)KNN–xBCZ] lead-free ceramics were prepared by the conventional solid-state method, and the effect of BCZ content on their phase structure and piezoelectric properties was studied. A coexistence of rhombohedral–orthorhombic phases was identified in the range 0.04<x<0.08. With increasing the BCZ content, their grain size becomes smaller, and their Curie temperature gradually decreases. An optimum piezoelectric behavior of d₃₃∼197 pC/N and k_p∼40.6% was demonstrated in the ceramic with x=0.06 because of the coexistence of two phases. As a result, the introduction of BCZ could further improve piezoelectric properties of KNN ceramics.     

Enhanced piezoelectric properties of (K0.40Na0.60)0.94Li0.06Nb0.94SbO3 lead-free ceramics by optimizing the sintering temperature

Effects of sintering temperature on the microstructure and electrical properties of (K_0.40Na_0.60)_0.94Li_0.06Nb_0.94SbO₃ (KNLNS) lead-free ceramics are investigated. The grain size gradually becomes larger with increasing sintering temperature from 1055 °C to 1105 °C, and the piezoelectric property could be enhanced by optimizing their sintering temperature. The ceramic sintered at 1075 °C has optimum electrical properties, i.e., d₃₃~272 pC/N, k_p~43.5%, ε_r~1152, tan δ~0.026, and T_C~346 °C. These results show that the sintering temperature can optimize electrical properties of KNLNS ceramics.     

An efficient approach for direct synthesis of K0.5Na0.5NbO3 powders

Pure K_0.5Na_0.5NbO₃ powders were prepared at low temperatures by an efficient method using Na₂CO₃, K₂CO₃ and Nb₂O₅ as raw materials and urea as fuel. The phase evolution of the powders was investigated by X-ray diffractometer (XRD) and thermo gravimetric analysis-differential scanning calorimeter (TG-DSC). The phase composition and morphology of the powders were characterized by electron probe microanalysis (EPMA) and scanning electron microscope (SEM), respectively. The results reveal that single-phase K_0.5Na_0.5NbO₃ powder can be obtained at 550 °C by the method. The as-prepared powder is stoichimetric, fine and well-developed.     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Phase transition behavior and electrical properties of lead-free (1 − x)(0.98K0.5Na0.5NbO3–0.02LiTaO3)–x(0.96Bi0.5Na0.5TiO3–0.04BaTiO3) piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(0.98K_0.5Na_0.5NbO₃–0.02LiTaO₃)–x(0.96Bi_0.5Na_0.5TiO₃–0.04BaTiO₃) (KNN–LT–BNT–BT) with x = 0–0.10 have been synthesized by a conventional sintering technique. All samples possess pure perovskite structure, showing room temperature symmetries of orthorhombic at x < 0.02, and tetragonal at 0.05 ≤ x ≤ 0.10. A coexistence of orthorhombic and tetragonal phases in the composition range of 0.02 ≤ x < 0.05 in this system is caused by the temperature of the polymorphic phase transition (PPT) decreasing to around room temperature but not the behavior of the morphotropic phase boundary (MPB). The samples near the coexistence region exhibit improved properties, which are as follows: piezoelectric constant d₃₃ = 155 pC/N, remnant polarization P_r = 24.2 μC/cm², and coercive electric field E_c = 2 kV/mm. The results indicate that although this kind of ceramics displays good properties, further study is needed to promote the stabilities of the ceramics in order to utilize them in varying temperature environments.     

Improved ferroelectric photovoltaic effect in Mn-doped lead-free K0.5Na0.5NbO3 films

The pure and Mn-doped K_0.5Na_0.5NbO₃ (KNN) films were deposited using solution-gelation method. The crystal structure, ferroelectric properties, spectral response and J-V performance of photovoltaic effect were systematically investigated. Both the ferroelectric and leakage properties are obviously enhanced for Mn-doped KNN films. A fascinating phenomenon is observed that the ferroelectric photovoltaic effect is enhanced in Mn-doped KNN films, which is originated from the improved ferroelectric polarization and narrower band gap. The transition element Nb partially substituted by Mn results in the lattice distortion and further destroys the symmetry space structure, which enhances ferroelectric polarization. And the narrower band gap effectively decreases the internal potential barrier to separate the carriers. This work gives a clear relationship between the lattice distortion, ferroelectric and photovoltaic response. It is certain that lead-free transparent K_0.5Na_0.5NbO₃ films can be potentially applied in viable ferroelectric based solar cells.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

An investigation of (Na0.5K0.5)NbO3–CaTiO3 based lead-free ceramics and surface acoustic wave devices

Lead-free (Na_0.5K_0.5)NbO₃ ceramics doped with CaTiO₃ (0–3 mol%) have been prepared by the conventional mixed oxide method in this paper. All of the CaTiO₃ doped (Na_0.5K_0.5)NbO₃ specimens do not deliquesce as exposed to water for a long time. The samples are characterized by X-ray diffraction analysis, Raman scattering spectra, scanning electron microscopy, and atomic force microscopy. The dielectric, piezoelectric and ferroelectric properties are also investigated. The results show that the addition of CaTiO₃ is very effective in preventing the deliquescence and in improving the electric properties of (Na_0.5K_0.5)NbO₃ ceramics. Finally, surface acoustic wave devices based on lead-free ceramics have been successfully fabricated and their characterization is presented.     

High piezoelectric coefficient of Pr2O3-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

Pr₂O₃-doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ-xPr) ceramics were prepared by the conventional solid-state method. A tetragonal phase is only observed in these ceramics, and the introduction of Pr₂O₃ decreases their sintering temperature without affecting negatively the piezoelectric constant. Enhanced ferroelectric properties were obtained in these BCTZ-xPr ceramics. The ceramic with x=0.06 wt% exhibits a good electrical behavior of d₃₃∼460 pC/N, k_p∼47.6%, ε_r∼4638, and tan δ∼0.015 when sintered at a low temperature of ∼1400 °C. As a result, the BCTZ-xPr ceramic is a promising candidate for lead-free piezoelectric ceramics.     

Effect of HfO2 content on the microstructure and piezoelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃–xHfO₂ [BNBT–xHfO₂] lead-free ceramics were prepared using the conventional solid-state reaction method. Effects of HfO₂ content on their microstructures and electrical properties were systematically studied. A pure perovskite phase was observed in all the ceramics with x=0–0.07 wt%. Adding optimum HfO₂ content can induce dense microstructures and improve their piezoelectric properties, and a high depolarization temperature was also obtained. The ceramics with x=0.03 wt% possess optimum electrical properties (i.e., d₃₃~168 pC/N, k_p~32.1%, Q_m~130, ε_r~715, tan δ~0.026, and T_d~106 °C, showing that HfO₂-modified BNBT ceramics are promising materials for piezoelectric applications.     

The effect of pre-annealing on the microstructure of (K,Na)NbO3 ceramics

The effects of pre-annealing on the microstructure development and piezoelectric properties for 0.95(K_0.5Na_0.5)NbO₃–0.05LiSbO₃ (0.95KNN–0.05LS) ceramics were investigated. The pre-annealing suppressed the abnormal grain growth in both the undoped and Mn-doped 0.95KNN–0.05LS ceramics. The pre-annealed samples possessed smaller abnormal grains, larger matrix grains, and a broader grain size distribution compared to the samples sintered without a pre-annealing step. The pre-annealed samples presented better dielectric and piezoelectric properties, a larger dielectric constant (ε_r) and electromechanical coupling factor (k_p), and a smaller dielectric loss factor (tan δ).     

Composite microstructures and piezoelectric properties in tantalum substituted lead-free K0.5Na0.5Nb1-xTaxO3 ceramics

K_0.5Na_0.5Nb_1-xTa_xO₃ (KNNT) (with x?=?0.00, 0.05, 0.10, 0.20, 0.30, 0.50 and 1) ceramics are prepared by ball milling and two calcinations at 830?°C for 5?h. Subsequent sintering of centimeter size pellets, 1–2?mm thick, is studied using conventional and spark plasma sintering techniques with various conditions. X-Ray diffraction and Raman spectroscopy phase identification reveal orthorhombic to tetragonal phase transitions occurring at about x?=?0.50, associated to chemical disorder. Scanning electron microscope observations and associated energy dispersive X-ray spectroscopy analysis reveal some composite aspect of the ceramics. Substitution of niobium by tantalum, corresponding to x increase, decreases significantly the grain size but also the densification of the ceramics sintered by conventional sintering, while, enhancement of the piezoelectric properties is observed for both sintering techniques. Thanks to parameters optimization of the spark plasma sintering process, temperature-time-pressure, significant improvement of the relative density over 96%, is obtained for all the compositions sintered between 920 and 960?°C, under 50?MPa, for 5–10?min with heating rates of 100?°C/min. High relative permittivity (ε_r =?1027), piezoelectric charge coefficient (d₃₃ =?160 pC/N) and piezoelectric coupling factor (k_p =?46%) are obtained in spark plasma sintered K_0.5Na_0.5Nb_1-xTa_xO₃ composite ceramics, for x ranging between 0.10 and 0.30 and for some specific spark plasma sintering conditions. Thus, tantalum single element substitution on niobium site, combined with spark plasma sintering, is revealed to be a powerful combination for the optimization and the reliability of piezoelectric properties in KNN system.     

Composition and poling condition-induced electrical behavior of (Ba0.85Ca0.15)(Ti1−xZrx)O3 lead-free piezoelectric ceramics

Lead-free (Ba_0.85Ca_0.15)(Ti_1−xZr_x)O₃ (BCTZ) piezoelectric ceramics were fabricated by normal sintering in air atmosphere. BCTZ ceramics with x = 0.10 possess a coexistence of tetragonal and rhombohedral phases at ∼40 °C. The Curie temperature of BCTZ ceramics decreases with increasing the Zr content. Piezoelectric properties of BCTZ ceramics are dependent on the poling conditions (i.e., the poling temperature and the poling electric field), and the underlying physical mechanism is illuminated by the phase angle. The BCTZ (x = 0.10) ceramic, which locates at the existence of two phases and is poled at E ∼ 4.0 kV/mm and T_p ∼ 40 °C, exhibits an optimum electrical behavior at a room temperature of ∼20 °C: d₃₃ ∼ 423 pC/N, k_p ∼ 51.2%, 2P_r ∼ 18.86 μC/cm², 2E_c ∼ 0.47 kV/mm, ?_r ∼ 2892, and tan δ ∼ 1.53%.     

CuO as a sintering additive for (Bi1/2Na1/2)TiO3-BaTiO3-(K0.5Na0.5)NbO3 lead-free piezoceramics

CuO as a sintering additive was utilized to explore a low-temperature sintering of 0.92(Bi_1/2Na_1/2)TiO₃-0.06BaTiO₃-0.02(K_0.5Na_0.5)NbO₃ lead-free piezoceramic which has shown a promise for actuator applications due to its large strain. The sintering temperature guaranteeing the relative density of greater than 98% is drastically decreased with CuO addition, and saturates at a temperature as low as ∼930 °C when the addition level exceeds ca. 1 mol.%. Two distinguished features induced by the addition of CuO were noted. Firstly, the initially existing two-phase mixture gradually evolves into a rhombohedral single phase with an extremely small non-cubic distortion. Secondly, a liquid phase induced by the addition of CuO causes an abnormal grain growth, which can be attributed to the grain boundary reentrant edge mechanism. Based on these two observations, it is concluded that the added CuO not only forms a liquid phase but also diffuses into the lattice. In the meantime, temperature dependent permittivity measurements both on unpoled and poled samples suggest that the phase stability of the system is greatly influenced by the addition of CuO. Polarization and strain hysteresis measurements relate the changes in the phase stability closely to the stabilization of ferroelectric order, as exemplified by a significant increase in both the remanent strain and polarization values. Electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the stabilization of ferroelectric order originates from a significant amount of Cu²⁺ diffusing into the lattice on B-site. There, it acts as an acceptor and forms a defect dipole in association with a charge balancing oxygen vacancy.     

Oxygen-enriched sintering for improved piezoelectric performance of (K0.5Na0.5)(Ta0.3Nb0.7)O3 lead-free ceramics: The impact of defects

In this work, (K_0.5Na_0.5)(Ta_0.3Nb_0.7)O₃ (KNNT) powders synthesized by the microwave-hydrothermal method were sintered under different oxygen partial pressure (P_O2 = 0, 0.498, 0.995?atm) atmospheres. Effects of P_O2 on the phase composition, microstructure, piezoelectric properties, and defects in the KNNT ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS). Results indicated that P_O2 had almost no effect on the phase structure and grain morphology of KNNT ceramics. Abnormal grain growth was observed with increasing P_O2; simultaneously, the density of the ceramics decreased. However, the d₃₃ value reached its maximum of 205 pC/N when sintered at the highest P_O2, and k_p showed a similar trend. TEM observations indicated that the microscopic domains had the most regular domain orientation at a P_O2 of 0.998?atm. These phenomena were due to the different oxygen vacancy concentrations caused by varying P_O2. UV–Vis DRS revealed that the defect concentration decreased with increasing P_O2; moreover, the oxygen vacancy concentrations were determined by XPS to be 23.5%, 21.1%, and 19.1% corresponding to P_O2 values of 0, 0.498, and 0.995?atm, respectively, indicating that oxygen-enriched sintering can reduce defects and enhance piezoelectric properties.     

Green and red upconversion luminescence of Er-doped K0.5Na0.5NbO3 ceramics

Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) lead-free piezoelectric ceramics were synthesized by the solid-state reaction method. The upconversion emission properties of Er³⁺ doped KNN ceramics were investigated as a function of Er³⁺ concentration and incident pumping power intensity. Bright green (~555 nm) and red (670 nm) upconversion emission bands were obtained under 980 nm excitation at room temperature, which are attributed to (²H_11/2, ⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. The upconversion emission intensity can be adjusted by changing Er³⁺ concentration, and the mechanism of upconversion processes involve not only a two-photon absorption but also a three-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its upconversion and piezoelectric property.     

Effects of CuO addition on the structure and electrical properties of low temperature sintered Ba(Zr,Ti)O3 lead-free piezoelectric ceramics

CuO-doped Ba(Zr_0.05Ti_0.95)O₃ (BZT) ceramics were prepared using conventional solid state reaction method, and their structure and electrical properties were investigated. It was found that a small amount of CuO could lower the sintering temperature significantly and make their microstructure more densified than pure BZT. The ceramics with 1.2 mol% CuO, sintered at 1250 °C, showed excellent piezoelectric properties with d₃₃~320 pC/N and k_p=44%. The sintering temperature was decreased by 150 °C than that for pure BZT ceramics while showing comparable piezoelectric properties. Moreover, the influence of sintering temperature on the optimally 1.2 mol% CuO-doped BZT ceramics was studied. With the temperature change, different patterns of crystal growth were observed in the doped BZT ceramics. When the sintering temperature increased from 1200 °C to 1350 °C, the patterns of normal–abnormal–normal grain growth were changed accordingly.     

Effect of Eu doping on structure and electrical properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

Eu-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT6-xEu, x=0.00–2.00 at%) lead-free piezoelectric ceramics have been synthesized by the solution combustion method. The effect of Eu doping concentration on the phase structure, microstructure and electrical properties of BNBT6 ceramics has been investigated. The XRD analysis confirms that the europium additive incorporates into the BNBT6 lattice and results in a phase transition from the coexistence of rhombohedral and tetragonal phases to a more symmetric pseudocubic phase. The SEM images indicate that the europium additive has little effect on the ceramic microstructure and the average grain size is about 2.0 μm. The electrical properties of BNBT6 ceramics can be improved by appropriate Eu doping. The 0.25 at% Eu doped BNBT6 ceramic presents excellent electrical properties: piezoelectric constant d₃₃=149 pC/N, remnant polarization P_r=40.27 μC/cm², coercive field E_c=2.95 kV/mm, dielectric constant ε_r=1658 and dissipation factor tan δ=0.0557 (10 kHz).